1. Field of the Invention
The present invention relates generally to measurement techniques. More particularly, the invention pertains to measurement techniques for determining the density of a substance having free water compensation. It has particular relation to the manufacturing of tobacco cigarettes where measurement techniques are employed in a continuous manufacturing process.
2. Description of the Prior Art
In the manufacturing industry, the need exists for determining the density of a substance as it is being processed on a real-time basis. Modern cigarette manufacturing machines, for example, produce a continuous paper and tobacco rod, which is cut into individual cigarettes by downstream apparatus. The density of the tobacco in the rod must be continuously measured, with the measurements directed back to the manufacturing machine for use in the control process of the machine.
Using radio frequency energy to measure the dielectric constant of small segments of the substance, such as the cigarette rod, and by knowing the physical properties of the cigarette rod's constituents, density profiles and weights can be calculated. An apparatus employing the above technique is disclosed in U.S. Pat. No. 5,698,986 issued to Mays et al.
The apparatus measures how the substance affects the resonance frequency of a microwave cavity. The resonance frequency of a microwave cavity depends on the size of the cavity and on the dielectric constant of the substance contained in the cavity. If the substance is a gas borne powder or mass of particles, the resonance frequency will be different from the resonance frequency determined when only the gas is present. This will in turn indicate the density of the powder, or the relative proportions of two powders if a mixture is introduced into the cavity, or any number of other desired measurements.
One source of error in this measurement technique is related to varying amounts of free water which may be contained within the substance. Accordingly, the need exists, in those applications where a substance may contain free water, to measure the free water content and to compensate the weight measurement.
Various methods for water measurement are also known in the art. One common method includes a source of microwave energy that is propagated through a particular substance and is attenuated by an amount dependent on the quality of moisture in the product. For example, U.S. Pat. No. 5,086,279 issued to Wochnowski et al. discloses a method and apparatus for measuring the moisture content of fibrous material in accordance with the previously described technique. U.S. Pat. No. 3,535,629 issued to Gibson et al. discloses another microwave moisture measuring apparatus. While systems such as those described above have provided some degree of success in measuring moisture, there still remains a need for improved systems. For example, in both systems described above, microwave energy is not contained within the system and is radiated outwardly which allows for potential interference with other RF systems, such as radio communication services. Still other known systems are not adapted for use in a continuous production system or are not designed or readily adapted for use with a cigarette manufacturing machine. Accordingly, the needs exists for a free water measuring device that can operate on a production line on a real-time basis with the density monitor of Mays et al.
As will be described in greater detail hereinafter, the method and apparatus of the present invention solves the aforementioned problems and employs a number of novel features that render it highly advantageous over the prior art.